Bolted joints connect separate pieces using a threaded hole and bolt, creating a strong connection due to increased surface area and friction. Vibration can cause loosening, but using washers and choosing the right bolt for the forces applied can prevent this.
A bolted joint is a type of threaded fastener that connects separate pieces together. These joints often look like a standard screw hole, even though the hole will extend through two or more pieces. When fully tightened, the bolted joint can withstand enormous amounts of pressure and torque without damage. This is the result of the forces between the bolt and the connected surfaces.
The main purpose of a bolted joint is to connect objects. This is typically achieved by using a threaded hole that sinks through one object and into the next. A bolt then screws into the hole and connects the pieces together. This connection is exceptionally strong, much stronger than other common forms of connection.
The strength of this connection comes primarily from the threads in the bore and on the bolt. These threads greatly increase the surface area between the bolt and the connected object. Since the connected surface area is very large, the amount of friction between the two objects increases. This means it requires more power to overcome the friction and get the bolt to move.
Because the threads run along the bolt shaft and hole, they stack on top of each other. This allows for a huge increase in surface area with very little space usage. Because so little space is used for such a large gain, it is possible to spread bolted joints over a greater distance.
The biggest common problem in bolted joints comes from vibration. When a bolted joint vibrates, the two materials and the bolt often vibrate at slightly different frequencies. This will cause the bolt to move slightly with each vibration. This movement is so slight that it is generally invisible to the naked eye and only becomes noticeable over time. As the vibration continues, the bolt will slowly loosen until the connection becomes unstable.
The most common method of dealing with this problem is to use a washer under the bolt head and any nuts on the fastener. The washers will help absorb a small amount of the vibratory motion and keep the seal tight longer. Additionally, a washer will provide a solid surface for the bolt or nut to rest on, allowing for a tight connection on contoured or soft surfaces.
A bolted joint works best when it is designed with the forces applied to it in mind. Certain types and styles of bolts work better under certain circumstances. If the fastener has less torque than it was designed for, the bolt will loosen over time. If it has more than it was made for, the additional torque can cause a shear force and break the bolt.
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